The present invention relates to recovering oil from subterranean oil reservoirs in which the reservoir oil contains petroleum acids which form surface-active soaps when reacted with an aqueous alkali. More particularly, the present invention relates to recovering oil from such a reservoir in which the soaps formed from the reservoir oil have only a relatively low extent of interfacial activity.
Numerous patents and journal articles relate to alkaline water-flooding processes ("caustic floods") in which alkaline aqueous fluids are injected into reservoirs to displace oil toward production locations. For example, U.S. Pat. No. 3,927,716 describes such a process in which the aqueous alkaline solution has a pH from about 11.5 to 13 and a relatively low monovalent salt concentration (from about 0.5 to 2.0 weight percent) and indicates that such proportions increase the oil displacing efficiency. U.S. Pat. No. 4,004,638 describes an alkaline water flooding process in which a slug of aqueous alkali is followed by a slug of aqueous alkali containing an organic sulfonate surfactant and indicates that such a procedure provides greater oil recovery. U.S. Pat. No. 4,081,029 describes an alkaline water flooding process using an alkali metal silicate solution and indicates that it is particularly advantageous where the oil is relatively viscous and/or the reservoir contains zones of significantly different permeability.
Such caustic flood oil recovery processes are complicated by the fact that as an aqueous alkali is flowing through a reservoir it tends to be consumed by physical or chemical interactions other than the desired conversion of petroleum acids to surfactant soaps. Many reservoirs contain multivalent cations dissolved in the reservoir water and/or adsorbed by cation exchange on clays, and such cations tend to react with and consume the caustic. In addition, some reservoirs contain silica compounds which react with and consume an aqueous hydroxide. Unless the caustic concentration in the injected solution is relatively high, most of the caustic is consumed before the solution flows through much of the reservoir.
Thus, the prior art teachings and beliefs indicate a problem. Although it is known that a low caustic concentration (generally, equivalent to about 0.2% by weight of sodium hydroxide) is desirable to cause the surfactant system containing the petroleum acid soaps formed by the caustic to have a low interfacial tension against the oil, a much higher caustic concentration (in the order of 1% by weight of sodium hydroxide) is needed to cause enough of the caustic to be effectively propagated through the reservoir. When a relatively concentrated alkaline solution is injected, the chemical costs are significantly increased, since more alkaline material is required and (due to the reduced efficiency) more solution is required. In addition, water-thickening chemicals may be needed to reduce the mobility of the alkaline solution. In general, the water-thickeners employed to provide a relatively low mobility aqueous solution suitable for an oil recovery process are polymers, such as the partially hydrolyzed polyacrylamides or polysaccharides such as the xanthan gum polymers. In addition to increasing the cost, the use of such polymers may introduce a chemical instability, due to its interactions with the alkaline solution and/or reservoir fluid. The rate and extent to which the polymers react with an aqueous alkali is known to increase with increases in the alkali concentration and temperature and such reactions may become significant at the temperature and pressure of the reservoir.
U.S. Pat. Nos. 3,653,440 and 4,203,491 by Joseph Reisberg describe oil recovery processes using slugs of preformed aqueous surfactant systems (which are effective or "active" in respect to reducing the interfacial tension between the surfactant system and the reservoir oil) displaced with a drive fluid comprising a substantially homogeneous mixture of aqueous liquid and gas, rather than an aqueous liquid containing a water-thickening polymer. In both of those patents, it is indicated that little or no oil-displacing effects were exhibited when such drive fluid dispersions (or foams) of gas and water were injected without the preceding slug of active surfactant.
The Ser. No. 369,449 patent application describes a process which is particulary effective for recovering oil from a reservoir in which the oil reacts with an injected aqueous alkali to yield soaps having a significant extent of interfacial activity which activity is optimum in the presence of relatively low concentrations of dissolved alkaline material. In that process the reservoir oil is displaced toward a production location by injecting an oil displacing mixture of gas and aqueous alkaline solution in which the proportions of dissolved alkaline material and neutral salt are arranged for substantially optimizing the activity of the surfactant system formed by the reaction with the reservoir oil. The disclosures of the Ser. No. 369,449 patent application are incorporated herein by reference.